Have you try to encapsulate the second call to translate inside another push/popMatrix?

Well the problem is that even this simple one

1. g.pushMatrix();
2. g.rotate(-rotation); // rotation!=0.0f
   
3. g.popMatrix();

does not show any images

What happens if you dont translate/scale and just rotate?

I am not sure to understand the problem. In particular, what you mean by " g is a PApplet".
Let's make a simple experiment:

1. void setup()
2. {
3.   size(800, 500);
4.   PImage img = loadImage("H:/Temp/paysage_typique_causse.jpg");
5.   pushMatrix();
6.   translate(width / 2, height / 2);
7.   rotate(-20);
8.   translate(-img.width / 2, -img.height / 2);
9.   image(img, 0, 0);
10.   popMatrix();
11. }

OK, the combined two translates centers the image, adding the rotate in the middle makes the image to be rotated in its center, at the center of the sketch.

Now, if I use the same instructions on a PGraphics, I get the same result:

1. void setup()
2. {
3.   size(800, 500);
4.   PImage img = loadImage("H:/Temp/paysage_typique_causse.jpg");
5.  
6.   PGraphics gr = createGraphics(width, height, JAVA2D);
7.   gr.beginDraw();
8.  
9.   gr.pushMatrix();
10.   gr.translate(width / 2, height / 2);
11.   gr.rotate(-20);
12.   gr.translate(-img.width / 2, -img.height / 2);
13.   gr.image(img, 0, 0);
14.   gr.popMatrix();
15.  
16.   gr.endDraw();
17.  
18.   image(gr, 0, 0);
19. }

Note that I deliberately avoided to name my PGraphics g, as it is the name of the PGraphics of the sketch, so there might be interferences (or not, but better safe than sorry...).
Note also that when using translate, I use 0, 0 coordinates, since positioning is done by translate().
Last remark: it is very easy to get nothing drawn with these instructions, as images can go beyond the sketch area...

Perhaps we can use this example as starting point, so you can show what is your exact problem.